Method of manufacturing electrical insulating structures



I N 1967 L. E. FEATHER ETAL. 3,351,693

METHOD OF MANUFACTURING ELECTRICAL INSULATING STRUCTURES 'Filed Feb. 14, 1964 Fig. 5

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Filed Feb. 14, 1964, Ser. No. 344,947; 2 Claims. (Cl. 264-160) This invention relates in general to a method of manufacturing electrical insulating structures and more particularly to a methodof manufacturing electrical insulating structures for use in inductive apparatus. I I

In the manufacture of electrical insulating structures for electrical coils, such as electrical pancake type coils for oil filled shell-form power transformers, .it is necessary to mold or form some parts from a fibrous, cellulosic material, such as pressboard. The outside corner channel, which is molded to conform to the contour of the corner and to enclose a predetermined portion'of the corner of a pancake type electrical coil, is an example. In forming the outside corner channel, a substantially rectangular pressboard blank of the desired dimensions and thickness is cut, and the forming, is performed in a hot pressing op eration. The depth of the channel and the radius of curvature of the base of the channel is usually such that softening of the pressboard is essential prior to themolding' operation. One method of softening the pressboard blank is to add water to the blank. The use of water, however, has the disadvantage of increasing the hot pressing time. It is imperative that substantially all of the water be removed in the hot pressing operation before the part is freed from the molding pressure, or the retained moisture will turn to steam and blister the part. If the amount of water is reduced in an effort to reduce pressing time, the pressboard will not be sufficiently softened, and a buildup in thickness will occur at the edge of'the channel legs. The build-up at the edge of the channel legs is due to wrinkling and folding of the inadequately softening pressboard during the hot-pressing step, and results in anonuniform buildup. The part may then'be unsuitableforuse.

Mechanical softening of the pressboardcan be. uniformly accomplished by a co'rrugating process. However, it has been found thatif the corrugations are made deep enough to provide sulficient' mechanical working to accomplish the 'desired softening, the length of the pressboardblank is shortened, and the effective thickness of the-'pressboard as it goes into 'the press is too great. If the corrugation is made more shallow, the-pressboard is not sufiiciently softened and folds and wrinkles are produced at the edges of the channel legs.

Accordingly, it is an object of this invention to provide a new and improved method of forming insulating structures forelectrical apparatus.

Another object of this. invention'is to provide a new and improved method of preparing insulating materials for forming which allows effective utilization of less costly materials.

A further object of this invention is to provide a new and improved method'of preparing insulating materials forforming which softens the material without substantially changing its dimensions;

Still another object of this invention is to provide a new and improved method of preparinginsulating materials for forming which requires a minimum press time and which eliminates the requirement of saturating the material with water prior to the press operation.

A further object of this invention is to provide a new and improved method of forming curved channels for inductive apparatus without folding and "wrinkling the'legs of the channels.

United States Patent O 3,351,693 Patented Nov. 7, 71967 Briefly, the present invention accomplishes the above cited objects by a method which utilizes mechanical working of the insulating material to obtain the desired degree of softening and flexibility. Inparticular, a blank of insulating material having the'desired dimensions is cut,

. and then corrugated as deeply as possble without rupture of the materiaL'The word corrugated as used herein means the process of producing spaced, parallel indentations having a uniform depth. The blank is corrugated again, only this time the corrugation depth is very shallow, with the second corrugating pattern being at a slight angle to I, the first corrugating pattern. The second corrugation will uniformly flatten the deeper first corrugation and return the insulation material to substantially the same dimensions and thickness of the blank prior to the first corruga-. tion. Thus, the blank of insulating material has beenmade very flexible due to the working of the material, without 1 and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

' For a better understanding of the invention reference may be had to the accompanying drawings in which:

FIGURE 1 is a plan view of an insulating blank after cutting from a sheet of insulating material;

FIGURE 2 is an enlarged fragmentary cross section line II-II of the insulating blank shown taken along the in FIGURE 1;

FIGURE 3 is a plan view showingthe insulating'blank v of FIGURE 1 after the first corrugation step;

1 in inductive apparatus, illustratingthe utilization'of the of flexibility and may be formed severely without fear of' FIGURE 4 is an enlarged fragmentary cross section taken along of the insulating blank shown in FIGURE 3 the line 1VIV; I

FIGURE 5 is a plan view showing the insulating blank of FIGURE 3 after the second corrugation step;

FIGURE 6 is an enlarged fragmentary cross section of the insulation blank shown in FIGURE 5, taken along the line VI-VI;

FIGURE 7 is a plan view of an outside corner channel for inductive apparatus; i

nel shown in FIGURE 7; and I FIGURE 9 is a plan view'of a pancake type coil utilized FIGURE 8 is an end view of the outside corner chanoutside corner channels shown in FIGURES 7 and 8.

Referring now to the drawings, and FIGURE 1 in particular, there is shown a typical insulation blank 11 subsequent to the blanking or cutting operation. As used in in- 1 ductive apparatus, the insulating structures are generally a formed of fibrous, cellulosic material having a thickness of .0625 to .1875 inch. This material is commonlyreferred to as pressboard in the electrical industry.

Pressboard having a high rag content has a high degree rupture. However, a pressboard having a high rag'content is expensive, and in order to reduce the cost of inductive apparatus, the less expensive types of pressboard are utilized-if possible. The less expensive types of pressboard are equal in electrical insulating strength to the more expensive types, but cannot be formed as severely without rupture. Therefore, when forming insulation structures such as outside corner channels for liquid-filled shell-form power transformers, which have a deep channel and small I bend radius, auxiliary means must be employed to obtain the desired flexibility and softness of the insulating blank prior to the actual forming operation. Saturating the pressboard with water produces a'plasticization which allows. forming without rupture, but has the serious drawback of' I greatly extending the press time. The press operation is;

generally performed under pressure at a temperature of approximately 150 C. Therefore, it is essential that substantially all of the water in the insulating material be removed before the mold pressure is terminated. Premature removal of mold pressure allows the retained water to turn to steam and blister the finished part, resulting in a costly reject. Mechanical working of the insulating material will break down interlaminar bonding and produce the necessary flexibility of the material. However, in order to work the material into the desired flexibility, it must be corrugated deeply, which adversely affects the dimensions of the blank and makes forming unpredictable, resulting in a high percentage of rejects due to insulating parts which are outside of the dimensional tolerances.

The process disclosed herein overcomes the above disadvantages by a mechanical working process which produces the desired degree of flexibility in the insulating blank without substantially changing the dimensions of the blank.

The first step of the process is to cut a blank 11, as shown in FIGURE 1. The blank 11 may have a configuration as shown in FIGURE 1, it may be rectangular, square, or any other configuration, depending upon the insulating structure to be formed. At this stage, the cross section of the blank 11 is uniform, with smooth upper and lower surfaces 10 and 12 respectively, as shown in FIGURE 2.

The next step is to corrugate the blank 11, by running the blank 11 through corrugating rolls. Machinery for producing spaced, parallel indentations 14 having a uniform depth, commonly called corrugations, is well known in the art. The corrugations 14 should be as deep as possible without rupturing the insulating material. The depth 16 of the corrugations 14, as shown in FIGURE 4, may be almost 50% of the thickness of the material without rupturing it. FIGURE 3 illustrates the appearance of the'blank 11 after the deep corrugation step. The deep corrugations 14 change the length of the blank 11 as shown at x, and a cross section as shown in FIGURE 4, taken along the line 4-4 in FIGURE 3, illustrates how the effective thickness of the blank has been increased. If the thickness of the blank 11 shown in FIGURE 2 is Z, the thickness of the blank after the deep corrugation may be increased as much as 1.5Z, as shown in FIGURE 4. If an insulating structure were to be formed from the blank as it appears in FIGURE 3, the results would be unpredictable, resulting in structures which would vary in size and produce a high percentage of rejects.

By adding another step to the preparation of the blank 11 prior to forming, however, it is possible to produce a blank 11 having a high degree of flexibility with dimensions that are substantially the same as the original blank 11 shown in FIGURE 1, and which will form in a predictable manner to the desired dimensions. This next step is to corrugate blank 11 again, only this time the corrugations 18 should be shallow. Also, the corrugations 18 should be at a slight angle to the deep corrugations 14 as shown in FIGURE 5, so that the deep corrugations 14 may be uniformly flattened. If the blank is to be bent, with its longitudinal dimension forming a radius, the highest degree of flexibility would occur with the deep corrugation being perpendicular to the longitudinal axis 20. FIGURES 3 and illustrate the deep corrugation 14 being almost perpendicular to the longitudinal axis 20. The shallow corrugation 18 is also almost perpendicular to the longitudinal axis 20, but arranged as shown in FIG- URE 5 to cross the deep corrugations 14 and uniformly flatten them. This particular configuration of the corrugations 14 and 18, however, is merely one of many which may be of many employed, and the invention is not limited to any specific pattern of deep and shallow corrugations.

The shallow corrugations 18 should be of sufficient depth to uniformly flatten the deep corrugations 14 and return the thickness of the material to substantially the same as the thickness Z. By substantially the same is meant the thickness should not exceed substantially l.lZ, as shown in FIGURE 6. It will be noted in examining FIGURE 6, that the depth 22 of the deep corrugations 14 has been substantially reduced. The shallow corrugations 18, while visible when observing the insulating blank 11 in the plan view of FIGURE 5, are too shallow to be readily apparent in the cross sectional view of FIGURE 6. 2 Blank 11, as it appears in FIGURES 5 and 6, has been uniformly softened and made flexible, and is now ready for the forming step which will produce an insulating structure of the desired shape and dimensions. An example of a diflicult shape to form is shown in FIGURES 7 and 8. The structure shown in FIGURES 7 and 8 is an outside corner channel 24 for protecting and insulating the outside corners of pancake type coils commonly utilized in liquid-filled shell-type power transformers. FIGURE 9 is a plan view of a pancake type coil 26, having a plurality of turns 28, with the outside corner channels 24 being shown in position.

The outside corner channel 24 is difficult to form because the part not only describes an are having outer and inner radii, 30 and 32, respectively, as shown in FIGURE 7, but it also must form a channel having a depth 34 and a Width 36, as shown in FIGURES 7 and 8, respectively. The bend radius 30 and channel depth 34 tend to fold and wrinkle the inner leg portions 38 of the channel, unless the insulating material is very flexible.

The method described herein, adequately softens and makes flexible the blank 11, and produces an outside corner channel 24 which is dimensionally accurate, free of blisters, and free of build up at the channel leg portions. A fine spray of water may be applied to the blank 11 just prior to the forming step if desired, without increasing the press time, as only the outer few thousandths of an inch will be wet. This provides additional plasticization if the pressboard is very dry.

Thus, in summary, the method proposed herein comprises the steps of cutting from a sheet of insulating material of the desired thickness a blank of insulating material having the desired dimensions, as shown in FIG- URES 1 and 2. Next, the blank is corrugated deeply as shown in FIGURES 3 and 4, such that the thickness of the blank is increased substantially 50%. Next, the blank 11 is corrugated shallowly, as shown in FIGURES 5 and 6, with the shallow corrugations being at a slight angle to the deep corrugations to uniformly flatten the blank 11 so that its thickness does not exceed its initial thickness by more than approximately 10%. The final step is the forming of the blank 11 to the desired structural shape, such as the outside corner channel shown in FIGURES 7, 8 and 9.

The single deep corrugation process for forming the outer corner channels shown in FIGURES 7, 8 and 9, used prior to the use of the invention disclosed herein, had a reject rate of 60-75% due primarily to dimensional inaccuracies. Since the disclosed process has been adopted, the reject rate has dropped to less than 5%. Thus, substantial savings have been produced without resorting to higher cost material, and without the necessity of saturating the material with water, which could require substantially longer press times.

While the structural shape described herein is an outside corner channel, it will be obvious that the method disclosed is applicable to the forming of any insulating structure which requires the blank to be flexible, and the advantages enumerated herein will be equally applicable to other insulating structural shapes.

Since numerous changes may be made in the above described method and diflerent embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative, and not in a limiting sense.

We claim as our invention:

1. A method of preparing electrical insulating material, comprising the steps of cutting a blank having predetermined dimensions from a sheet of insulating material of predetermined thickness, corrugating said blank to form a plurality of spaced parallel first indentations having a uniform depth which increases the effective thickness of said blank by substantially fifty percent and which shortens a predetermined dimension of said blank, and corrugating said blank to form a plurality of spaced parallel second indentations having a uniform depth substantially less than the depth of said first indentations, said second indentations crossing said first indentations at an angle which returns said blank to within ten percent of its original predetermined thickness and to substantially its original predetermined dimensions and then forming said blank to construct an electrical insulating structure for electrical inductive apparatus having a predetermined configuration and dimensions.

2. The method of claim 1 wherein the angle at which said first and second indentations cross each other is such that the angle between the first indentations and the predetermined shortened dimension of said blank is substantially equal to the angle between the second indentations and the predetermined shortened dimension of said blank.

References Cited UNITED STATES PATENTS 1,748,389 2/1930 Marcel 264-286 2,721,819 10/1955 Munro 161131 3,058,160 10/1962 Mocker 264-294 3,067,806 12/ 1962 Trelease 264286 3,189,681 6/1965 Feather 264-320 ROY B. MOFFITI, Primary Examiner.

ROBERT F. WHITE, Examiner.

R. R. KUCIA, Assistant Examiner. 

1. A METHOD OF PREPARING ELECTRICAL INSULATING MATERIAL, COMPRISING THE STEPS OF CUTTING A BLANK HAVING PREDETERMINED DIMENSIONS FROMA SHEET OF INSULATING MATAERIAL OF PREDETERMINED THICKNESS, CORRUGATING SAID BLANK TO FORM A PLURALITY OF SPACED PARALLEL FIRST INDENTATIONS HAVING A UNIFORM DEPTH WHICH INCREASES THE EFFECTIVE THICKNESS OF SAID BLANK BY SUBSTANTIALLY FIFTY PERCETN AND WHICH SHORTENS A PREDETERMINED DIMENSION OF SAID BLANK, AND CORRUGATING SAID BLANK TO FORM A PLURALITY OF SPACED PARALLEL SECOND INDENTATIONS HAVING A UNIFORM DEPTH SUBSTANTIALLY LESS THAN THE DEPTH OF SAID FIRST INDENTATIONS, SAID SECOND IN- 